1. Field of the Invention
The present invention relates to a laser processing method and a laser processing system.
2. Description of Related Art
It is well known that a laser beam can propagate a long distance without spreading compared with natural light. Therefore, a laser can make a hole by focusing it, in a metal sheet or the like which is placed more than 10 km away. But to make this hole, a high-power laser which has a good air transmissivity, without the laser being absorbed by nitrogen, oxygen and water vapor, is necessary. Such a laser is a solid state laser (i.e. an Nd:YAG laser, a fiber laser or the like) operating at an IR region or an iodine laser.
Regarding the above two types of lasers, the iodine laser, which is sometimes called COIL (Chemical Oxygen Iodine Laser), is well known to be able to operate at a high power CW (continuous wave) mode with a wavelength of 1.315 μm. In order to operate the COIL, singlet oxygen molecule (O2(1Δg)) is generated from the chemical reaction of chlorine gas with a BHP solution which is a mixed solution of hydrogen peroxide solution (H2O2) and potassium hydroxide (KOH) or sodium hydroxide (NaOH). By transferring the energy of O2(1Δg) to a basic iodine atom (I) (i.e. by producing I(2P3/2) in an exited state from I(2P1/2) in a ground state), the laser can be operated. “Stephen C. Hurlick, et al., “COIL technology development at Boeing,” Proceedings of SPIE Vol. 4631, 101-115 (2002)”, “Masamori Endo, “History of COIL development in Japan: 1982-2002,” Proceedings of SPIE Vol. 4631, 116-127 (2002)”, “Edward A. Duff and Keith A. Truesdell, “Chemical oxygen iodine laser (COIL) technology and development,” Proceedings of SPIE Vol. 5414, 52-68 (2004)” and “Jarmila Kodymova, “COIL—Chemical Oxygen Iodine Laser: advances in development and applications,” Proceedings of SPIE Vol. 5958, 595818 (2005)” explain about the iodine laser.
Conventionally, it is difficult to have the laser propagate to a target placed more than 10 km away even with a laser having good air propagation characteristics if there is a cloud or a fog in the air. In other words, as a cloud or a fog is a cluster of water molecules which accumulate together and become enormous, a laser beam is scattered by the cluster. When there is a fine particle which becomes a core, the molecules of water are clustered. The core of the cluster is sometimes referred to as aerosol. Alternatively, the cluster of water molecules may be referred to as aerosol
Therefore, research was performed to improve the air transmissivity by using a laser which can vaporize a cloud or a fog. According to “REPORT SRL 02-F-1989, “LASER PULSE FORMATTING TO REDUCE THERMAL BLOOMING BY AEROSOL VAPORIZATION,” FINAL TECHNICAL REPORT, 17 Jan. 1989”, a KrF excimer laser can vaporize a cloud or a fog since its beam has a good absorption in regard to aerosol. A laser which can vaporize aerosol is referred to as LAV (Laser for Aerosol Vaporization).